1. Field of the Invention
The invention relates to a multiple-screw extruder. In particular, the present invention relates to a multiple-screw extruder with a planetary gear and to a method of processing and treating high-viscosity media using a multiple-screw extruder according to the invention.
2. Description of the Related Art
Conventional apparatuses for the treatment or production of plastics materials in the molten phase are, for example, containers which are arranged vertically or horizontally having stirrer mechanisms, extruders with one or two shafts, or thin-layer apparatuses. In particular at high viscosities, i.e., in the final stage of polymer production, such arrangements have a few known disadvantages, such as poor and irregular flow behavior (broad residence time distribution), settling of product on walls, covers and in dead corners. Additionally, these arrangements suffer from a slow reaction rate, particularly in the case of polycondensation reactions, for example polyester production, because the available surface for liberation of low molecular weight gaseous by-products is small, is inaccessible for a vacuum, or is sparingly renewed.
For the expounded reasons there has been made efforts to overcome the faults of the conventional devices by means of multiple-shaft reactors.
One of the first of these apparatuses is described in DE-PS 15 45 209. This patent discloses a horizontal reactor with several parallel, closely intermeshing, single-start screws which rotate in the same direction and are immersed above an axis in a pool of melt. Although backmixing and settling in the part of the reactor contacted by the melt may be avoided by forced conveyance, the specific surface area of the pool of melt and, therefore, the reaction rate are still small and slow, respectively. Consequently, deposits of desublimated oligomers on the cover cannot be avoided in the vapor chamber of the reactor.
Improvement has been made with a multiple-shaft kneading disc reactor according to DE-PE 30 30 541, in which several axially parallel intermeshing screw shafts with conveying kneading discs are arranged in a circle around a degasification chamber which faces thin films of the polymer melt that are formed on the screws. Stationary shafts are all guided individually through a housing to the gear which, in practice, leads to problems with sealing the reaction chamber if a fine vacuum of up to 10.sup.-4 bar is to be maintained, for example during polyester production. Sealing of this type of reactor is however problematic and demands a correspondingly high expenditure.
Solutions to the sealing problem were subsequently proposed by using only a main driving shaft which is guided through the housing wall and sealed. Such an apparatus is described in EP-OS 0 215 460. This disclosed apparatus is a vertical thin-layer apparatus in which several shafts are caused to roll in a planetary fashion on a stationary central gear-wheel. The shafts perform a revolving movement and a rotational movement in the same direction and spread a film of melt on the internal cylinder wall at high relative velocity. A problem associated with this apparatus resides in the one-sided guidance and low-friction mounting of the shaft ends in perforated discs connected to the driving shaft. The shafts are designed as grooved rods which are spaced at a certain distance from one another. Cleaning is considerably poorer than with meshing screw shafts, and uniform conveying behavior over the cross section, which is a condition for a narrow residence time distribution, is not ensured.
A multiple-screw extruder according to DE-PS 40 01 986 has the same basic features of only one central driving shaft guided from a housing and having circling planetary shafts. By way of a helical-geared central spindle in the gearing, the planet spindles arranged in a circle around the spindle can easily be driven because they are also helical-geared and, therefore, roll on the central spindle and also in the teeth of the internal housing wall. As the central spindle rotates, the planetary shafts consequently perform a rotational and a simultaneous revolving movement on the internal housing wall. Intermeshing screws are provided between the planetary roller portions while the internal housing wall and the central shaft are smooth in the processing section. The geometric conditions of the toothed gearing of this multiple-screw extruder, which are in contrast to the thin layer apparatus described in EP-OS 0 215 460, cause the direction of rotation of the planetary shafts to be contrary to the direction of revolution. This means that the relative velocity of the passing screw webs and, therefore, their conveying effect, are very slight with regard to the internal housing wall and the surface of the rotating central shaft in the processing section. In comparison with the multiple-shaft kneading disc reactor disclosed in DE-PS 30 30 541, this fact is a considerable disadvantage with respect to self-cleaning and uniform conveying behavior. Furthermore, holdup and surface renewal --decisive parameters in diffusion-controlled processes--can only be calculated with difficulty.